Why greasy foods make paper bags translucent

You might have wondered why a cheesesteak bag or a pizza box
becomes slightly see-through when touched by its greasy contents.

To understand why, we need to understand how light interacts with
matter. The colors we see are different energies of visible light
waves. We see these waves as the different colors of the rainbow
— red, orange, yellow, green, blue, and purple.

When a light wave hits an object, a few things could happen,
depending what the object is made of. The wave could be absorbed
by the object, it could be reflected (light bounces off at the
same angle it hit), it could be scattered (bounced around or
reflected in many different directions), it could be refracted
(bent), or transmitted (passed through making the object
transparent).

The color of the object that we see is the color of light that is
reflected. A banana is yellow because yellow light is reflected
back to our eyes and other wavelengths of light are absorbed.

Snow is white because it reflects and scatters all the different
colors of light equally. Snow is made of ice crystals with
tiny pockets of air between those crystals. When light hits snow,
the light is scattered and reflected as it passes through all
these different crystals.

The same thing happens when light hits a piece of paper. Paper is
made of fibers and there are little pockets of air between those
fibers. When oil, grease, or fat comes in contact with paper,
tiny droplets of it fill all the little gaps between the fibers
of the paper.

Similarly, ice appears transparent because it does not have those
pockets of air so light goes right through.

The scattering of light by an object like a paper fiber depends
on its size and shape, but also on the difference of the amount
of light that's refracted, known as the index of refraction,
between the fibers and its
surroundings, explains Michael Patterson,
a professor of physics at McMaster University in Ontario.
Generally, the smaller the difference, the less scattering.

"The oil, grease, or fat has about the same index of refraction
as paper," says Scheckel. "So the amount of scattering is kept to
a minimum. Most of the light that would be scattered from the
not-oiled paper is now transmitted through the paper."

Water has a lower index of refraction than paper fibers, which is
why it generally does not make paper transparent.